1. Field of the Invention
The invention relates generally to oil and gas wireline operations. More specifically, the invention relates to an apparatus for protecting a wireline from damage during a fishing operation.
2. Background Art
In oil and gas wireline operations, downhole tools, e.g., logging tools, are conveyed into and withdrawn from a wellbore using an armored electrical cable called a wireline. The downhole tools are typically tubular members that are threaded together to form a xe2x80x9ctool string.xe2x80x9d A cable head couples the wireline to the tool string. Occasionally, during operation, the tool string may become stuck in the wellbore. When the tool string gets stuck, a high tension is usually applied to the tool string to try to free the tool string from its stuck position. This high tension is applied to the wireline at the surface by a winch used to extend and retract the cable, and the wireline transmits the applied tension to the cable head. The cable head in turn transmits the tension to the tool string. The amount of tension available to free the tool string from its stuck position depends on the breaking strength of the cable, the profile and coefficient of friction of the wellbore, the position of the tool string inside the wellbore, and various other parameters, in particular the weight of the cable in the wellbore.
The connection between the cable head and the wireline typically includes a xe2x80x9cweak point.xe2x80x9d A weak point is a link designed to break when a predetermined amount of tension is applied to it. Normally, the weak point has the lowest breaking strength in the tool string. The weak point allows the cable to be separated from the cable head in the event that enough tension cannot be applied to the tool to free it. In situations where the tension required to free the tool string exceeds the breaking strength of the cable, the tool string must be xe2x80x9cfishedxe2x80x9d out of the wellbore. To fish the tool string, the operator first latches onto the cable head or tool string using a fishing tool coupled to one end of a drill pipe and then applies tension to the wireline to break the weak point and release the wireline from the cable head. The wireline is first removed from the wellbore, and then the cable head and the tool string are pulled out of the wellbore by removing the drill pipe.
In a method of fishing known as the cut and thread method, the cable is used to guide the fishing tool to the stuck tool string. FIG. 1 shows a prior art setup from the rig floor up for a fishing operation. During normal wireline operations, the cable 2 is payed from a cable drum (not shown). The cable 2 is threaded through sheaves 4, 6 and then passed through the rotary table 8 into the wellbore (not shown). In order to allow the cable 2 to guide the fishing tool (not shown) to the stuck tool (not shown) in the wellbore (not shown), the cable 2 is cut near the rig floor. An overshot 10 is connected to the winch-side section 2a of the cable 2 threaded through the sheaves 4, 6, and a spearhead 12 is connected to the downhole-side section 2b of the cable 2 that extends through the rotary table 8. In the illustration shown, a drill pipe 14 is supported by slips (not shown) in the rotary table 8. The fishing tool (not shown) is attached to the end of this drill pipe 14. A clamp 15 is then mounted on the upper end of the drill pipe 14 to hold the spearhead 12 and prevent the spearhead 12 from falling through the drill pipe 14.
The next step is to make up a drill pipe stand 16 and then connect the drill pipe stand 16 to the drill pipe 14. Typically, the drill pipe stand 16 consists of three drill pipe xe2x80x9cjointsxe2x80x9d threaded together. After connecting the drill pipe stand 16 to the drill pipe 14, the drill pipe stand 16 is lowered through the rotary table 8 into the wellbore (not shown). This process of connecting a drill pipe stand to the uphole end of the drill pipe at the rotary table 8 and lowering the drill pipe stand through the rotary table 8 is repeated until the fishing tool (not shown) on the end of the drill pipe 14 reaches the stuck tool (not shown) in the wellbore (not shown). In the illustration, a drill pipe stand 16 is suspended above the drill pipe 14 by a power drive 18. The power drive 18 is supported by the derrick 20 and rides on the guide rail 22. Before connecting the drill pipe stand 16 to the drill pipe 14, the winch-side cable section 2a is threaded through the drill pipe stand 16 until the overshot 10 engages with the spearhead 12. After the tension on the cable 2 is increased, the clamp 15 is then removed to allow the drill pipe stand 16 to be connected to the drill pipe 14. The power drive 18 is used to lower the drill pipe 14 having the new stand 16 connected thereto.
FIG. 2 shows the drill pipe stand 16 connected to the drill pipe 14. To connect the drill pipes, the drill pipe stand 16 is rotated relative to the drill pipe 14 so that the threads in the bottom drill pipe 16a can engage with the threads in the drill pipe 14. During this operation, the cable 2 is held in tension. The winch-side cable section 2a is also slanted with respect to the upper drill pipe 16b because the sheave 6 must be hung off to the side to give the power drive 18 room to operate. During normal wireline operations, the sheave 6 is typically held directly above the rotary table 8. With the winch-side cable section 2a in this slanted position, the threads in the upper drill pipe 16b contact the wireline section 2a as the upper drill pipe 16b rotates. The effect is that the threads cut into the winch-side cable section 2a, reducing the tensile strength of the cable 2 at the contact area. As more stands of drill pipe are made up and connected to the drill pipe 14 in the wellbore using the process described above, the threads on the drill pipe repeatedly contact the winch-side cable section 2a in the same area, further weakening the cable 2 at the contact area. As the degradation continues, a point is reached where the cable 2 breaks. Thus, a mechanism is needed for protecting the cable 2 while connecting stands of drill pipe to the drill pipe in the wellbore.
In one aspect, the invention relates to a cable protector which comprises a housing body having a central bore. The housing is adapted to fit in a pipe connector. An insert body is rotatably supported in the central bore. The insert body has a groove which is adapted to receive a cable.
In some embodiments, a roller is rotatably supported in the groove. The cable rides on the roller when there is relative movement between the cable and the housing body. In some embodiments, a wall of the housing body includes an opening which provides access to the central bore of the housing body. In some embodiments, the housing body has a threaded end portion for engagement with a threaded pipe.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.